Calculating bond energy

Worked example 3

Bond breaking is an endothermic process. 

Bond making is an exothermic process. 


Overall energy change =  breaking bonds total energy –  making bonds total energy 

Worked example 3:

Methane (CH4) reacts with chlorine (Cl2) 

to form chloromethane (CH3Cl) and hydrogen chloride (HCl). 

Calculate the overall energy change for the reaction above. (6)

The word equation is: 

Methane               +      Chlorine      -------->       Chloromethane    +     Hydrogen chloride

The symbol equation showing all the bonds is: 

Step 1. Calculate energy taken in when breaking bonds (1) 

(4 x C - H) + (1 x Cl - Cl)

(4 x 412) + (1 x 242) = 1648 + 242 = 1890

Step 2. Calculate energy released from making bonds (1) 

(3 x C-H) + (1 x C - Cl) + (1 x H - Cl)

(3 x 412) + (1 x 328) + (1 x 431) = 1236 + 328 + 431 = 1995

Step 3: Calculate total energy change (1) 

Overall energy change =  breaking bonds total energy –  making bonds total energy 

Overall energy change = 1890 - 1995

Overall energy change = – 105 kJ/mol 

Step 4: Explain if the reaction is an endothermic or an exothermic process. (3)

Overall energy change = – 122 kJ/mol